The present invention relates to cellular radiocommunications using macrodiversity techniques.
The infrastructure of a cellular network comprises base stations distributed over the covered territory for communicating with mobile stations located in the zones, or cells, that they serve. The macrodiversity technique consists in providing for a mobile station to be able to communicate simultaneously with separate base stations in such a way that, in the descending direction (from the base stations to the mobile stations), the mobile stations receive the same information several times and, in the ascending direction, the signal transmitted by the mobile station is picked up by the base stations in order to form different estimates that are then combined in the network infrastructure.
Macrodiversity procures increased reception that improves the performance of the system due to the combination of different observations of a same information item. It also makes it possible to carry out soft intercellular transfers (“soft handoff”) when the mobile station is moving.
Cellular networks can comprise sectored cells, in which the base station has a group of antennas arranged to transmit different radio signals in different directions defining the sectors of the cell. Macrodiversity can also be provided between several sectors of a same cell, the mobile station then receiving separate signals transmitted from the same base station. It is then referred to as “softer handoff” instead of “soft handoff” (see C. C. Lee and R. Steele, “Effect of Soft and Softer Handoffs on CDMA System Capacity”, IEEE Transactions on Vehicular Technology, Vol 47, No3, August 1998, pages 830–841). For the purposes of the present application, the term “base station” will designate either the base station of a non-sectored cell, or the means that a base station uses for defining one of the sectors of a sectored cell.
Macrodiversity techniques are particularly used in Code Division Multiple Access (CDMA) networks. They are provided in the third generation cellular system called UMTS (“Universal Mobile Telecommunications System”), in the context of wide band CDMA (W-CDMA) for frequency duplex communications (FDD). UMTS has been adopted in its general principles by the ETSI (European Telecommunications Standard Institute), and proposed to the International Telecommunications Union (ITU-R) for standardisation. The ETSI is distributing detailed documentation about it “The ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT Candidate Submission” on the Internet (http://www.etsi.org/smg). The partnership organisation for the third generation system (3 GPP) is distributing technical specifications on the Internet (http://www/3gpp.org) which relate to the UMTS system. These technical specifications use a proper vocabulary currently used in actual implementation projects.
For example, a base node (B node) groups one or more base stations of a same cell. A mobile station is called “User Equipment” (UE). A “Radio Network Controller” (RNC) manages one or more base nodes by means of base utilisation interfaces (lub). For a given communication, a “Service Radio Network Controller” (SRNC) communicates with a UMTS switch by means of a utilisation interface (lu). Another radio network controller (DRNC) can also communicate with the first radio network controller by means of network utilisation interfaces (lur).
The invention more particularly relates to a use of macrodiversity on the uplink. The uplink denotes the communications from the user equipment to the cellular network infrastructure. When several base nodes each receive a radio signal value carrying a same information item transmitted by a user equipment, there is macrodiversity on the uplink.
A use of macrodiversity on the uplink gives rise to specific problems that do not arise with a use of macrodiversity on the downlink, that is to say from the cellular network infrastructure to the user equipment.
In the user equipment, the radio signals coming from several base nodes are available locally in order to be combined in such a way as to increase the reception quality.
When several nodes each receive a radio signal value carrying a same information item transmitted by a user equipment, it would be possible to think of gathering the various values of this signal at a same point where they would be combined, in a way similar to that which is done in a user equipment for the downlink. However, using the cellular network infrastructure for systematically gathering these various values necessitates sizing the network with a sufficient bandwidth for transferring the various values of the radio signal to a same point, to combine them in order to obtain a communication signal and to transfer the communication signal obtained to its recipient. The problem is amplified by the fact that several user equipments must be able to transmit several radio signals to one or more base nodes, thus giving rise to a considerable increase in traffic on the network of the cellular network infrastructure.